If you ever used fiber cables you might have noticed that there is a different connector to transmit and receive traffic.

If one of the cables (transmit or receive) fails we’ll have a unidirectional link failure and this can cause spanning tree loops. There are two protocols that can take care of this problem:

• LoopGuard
• UDLD ( Unidirectional Link Detection )

Let’s start by taking a close look at what will happen if we have a unidirectional link failure:

Imagine the links between the switches are fiber links. In reality there’s a different connector for transmit and receive. SW3 is receiving BPDUs from SW2 and as a result the interface has become an alternate port and is in blocking mode.

Now something goes wrong…the transmit connector on SW2 towards SW3 was eaten by mice failed due to unknown reasons. As a result SW3 is not receiving any BPDUs from SW2 but it can still send traffic to SW2.

Because SW3 is not receiving anymore BPDUs on its alternate port it will go into forwarding mode. We now have a one way loop as indicated by the green arrow.

One of the methods we can use to solve our unidirectional link failure is to configure LoopGuard. When a switch is sending but not receiving BPDUs on the interface, LoopGuard will place the interface in the loop-inconsistent state and block all traffic:

Let’s take a look what this looks like on actual switches. I will use the same topology:

Let’s enable loopguard:

SW1(config)#spanning-tree loopguard default

SW2(config)#spanning-tree loopguard default

SW3(config)#spanning-tree loopguard default

Use the spanning-tree loopguard default command to enable LoopGuard globally. I don’t have any fiber connectors so I’m unable to create a unidirectional link failure. I can simulate it however by using BPDUfilter on SW2’s fa0/16 interface. SW3 won’t receive any BPDUs anymore on its alternate port which will cause it to go into forwarding mode:

SW2(config)#interface fa0/16
SW2(config-if)#spanning-tree portfast trunk
SW2(config-if)#spanning-tree bpdufilter enable

Here’s what will happen:

SW3# 
*Mar  1 00:17:14.431: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port FastEthernet0/16 on VLAN0001.

Normally this would cause a loop but luckily we have LoopGuard configured. You can see this error message appearing in your console, problem solved!

hostname SW1
!
spanning-tree loopguard default
!
end

hostname SW2
!
spanning-tree loopguard default
!
interface FastEthernet0/16
 spanning-tree portfast trunk
 spanning-tree bpdufilter enable
!
end

hostname SW3
!
spanning-tree loopguard default
!
end

If you want you don’t have to configure LoopGuard globally, you can also do it on the interface level like this:

SW3(config-if)#spanning-tree guard loop

The other protocol we can use to deal with unidirectional link failures is called UDLD (UniDirectional Link Detection). This protocol is not part of the spanning tree toolkit but it does help us to prevent loops.

Simply said UDLD is a layer 2 protocol that works like a keepalive mechanism. You send hello messages, you receive them and life is good. As soon as you still send hello messages but don’t receive them anymore you know something is wrong and we’ll block the interface.

Let’s use the same topology but configure UDLD this time. Don’t forget to get rid of loopguard first…

SW1(config)#udld ?      
  aggressive  Enable UDLD protocol in aggressive mode on fiber ports except
              where locally configured
  enable      Enable UDLD protocol on fiber ports except where locally
              configured
  message     Set UDLD message parameters

There are a number of methods how you can configure UDLD. You can do it globally with the udld command but this will only activate UDLD for fiber links!